1. Field of the Invention
The present invention relates to a nonreciprocal circuit device, such as an isolator, a circulator, etc., for use in a microwave band.
2. Description of the Related Art
In general, lumped-constant-type isolators for use in mobile communication apparatuses, such as portable telephones, allow a transmission signal to pass only in the transmission direction and prevent transmission thereof in the reverse direction. Also, recently there has been a strong demand for mobile communication apparatuses to have a lower cost as well as a smaller size and a lighter weight to make them easier to use, and in response to this, a smaller size, lighter weight, and lower cost isolator is also in demand.
A conventional lumped-constant-type isolator has a construction in which, as shown in FIG. 20, a permanent magnet 52, a center electrode body 53, a matching circuit board 54, and a grounding plate 55 are disposed in sequence from the top between upper and lower yokes 50 and 51. The center electrode body 53 is constructed in such a way that three center electrodes 57 are placed on a circular-plate ferrite 56 so as to intersect each other in an electrically insulated state.
The matching circuit board 54 has a circular hole 54b through which the center electrode body 53 is inserted. The circular hole 54b is formed in the central portion of a dielectric substrate 54a in the form of a rectangular thin plate. Around the edge of the circular hole 54b of the dielectric substrate 54a capacitor electrodes 58 are formed to be connected to input/output ports P1 to P3 of each of the center electrodes 57. A termination resistance film 59 is connected to the port P3.
In this conventional matching circuit board 54, the circular hole 54b must be formed and each capacitor electrode 58 must be formed as a pattern on the dielectric substrate 54a. Therefore, processing during manufacture and handling during assembly take time and effort, presenting the problem that the costs are increased.
Also, in the conventional matching circuit board 54, portions other than the capacitor electrodes 58 cause an increase in area and an increase in weight, presenting the problem that the above-described demand for a smaller size and lighter weight isolator cannot be met. In this regard, in recent isolators, there has been a demand for reduction in weight on the order of milligrams.
Instead of the above-described matching capacitor on a matching circuit board, it is possible to employ a single-board-type capacitor wherein capacitor electrodes are formed on the entire surface of both sides of a dielectric substrate with the board in between.
This single-board-type capacitor can be manufactured merely by forming electrodes on both main surfaces of a motherboard made of a large flat plate and by cutting the motherboard to predetermined dimensions, and mass production thereof is possible. For this reason, compared to a conventional case in which circular holes and a Plurality of capacitor electrodes are formed on a dielectric substrate, processing and handling are easy, and costs can be reduced. Also, since electrodes are formed on the entire surface of the board, a wasteful increase in area and in weight can be eliminated, and a smaller size and a lighter weight can be achieved by a corresponding amount.
FIGS. 16 to 19 show an example of an experimental unpublished isolator employing the single-board-type capacitor. In the figures, the reference 10 numerals which are the same as those of FIG. 20 indicate the same or corresponding components. This isolator is constructed such that a circular hole 61 through which a center electrode body 53 is inserted is formed on a bottom wall 60a of a grounding member 60 made of a resin, and single-board-type capacitors C1 to C3 and a single-board-type resistor R are disposed in such a manner as to surround the center electrode body 53 around the edge of the circular hole 61.                A grounding electrode 63 formed in the grounding member 60 is connected to a capacitor electrode 62 on the cold side (the bottom surface) of each of the single-board-type capacitors C1 to C3, and the input/output ports P1 to P3 of each center electrode 57 are connected to the capacitor electrode 62 on the hot side (the top surface).        
Here the cold side means one side of a capacitor to be connected to a grounding electrode and the hot side means another side of the capacitor to be connected to a port electrode (i.e., a signal line.)
In the single-board-type capacitors C1 to C3, the capacitor electrode 62 is positioned up to an edge 64a of a dielectric substrate 64 as shown in FIG. 19. When the entire surface of the capacitor electrode 62 is soldered and connected to the grounding electrode 63, thermal stress due to a difference in the thermal expansion coefficients between the dielectric substrate 64 and the grounding electrode 63 is likely to concentrate in the capacitor electrode 62 at the portion near this edge 64a and then may cause the capacitor electrode 62 to be peeled off.
When, in particular, the capacitor is employed in an isolator, heat is generated during transmission as 10 a result of insertion loss and consumption of reflected power at the termination resistor. Further, when the motherboard is cut, very small cracks are likely to be generated in the vicinity of the end surface of the capacitor. This also may cause the electrode peeling. During reception, on the other hand, when the capacitor is subjected to a thermal cycle, such as by being cooled again, the problem with electrode peeling is likely to occur.